Abstract:A series of C1 symmetric, 1,2′-bridged bis(indenyl)zirconium dichlorides were prepared to study the effect of ligand substitution, symmetry, and bridge identity on the stereoselectivity of propylene polymerization. Unsubstituted [1-(1-indenyl)-2-(2-indenyl)ethane]zirconium dichloride, Et(2-Ind)(1-Ind)-ZrCl 2 (1), was synthesized, and its propylene polymerization behavior was compared to three 2-phenylsubstituted complexes with different bridges: (4). The polymerization activity, polypropylene molecular weight,… Show more
“…Kaminsky pointed out that the angle between the bridged ligand and its various substitutions can play a pivotal role for activity and stereospecificity. 15 This was verified by the experimental result that Et(2-Ind)(2-Ph-1-Ind)ZrCl 2 (Et = C 2 H 4 ) gave polypropylenes with higher isotacticity 16 ([mmmm] = 74%) than anti C 2 symmetric Me 2 Si(2-Ph-1-Ind) 2 ZrCl 2 ([mmmm] = 58%).…”
The [SiH 2 (Ind) 2 ZrCH 3 ] + (Ind = indenyl) catalyzed stereoregularity of propylene polymerization mechanism has been investigated at M06 level of theory. Four different approaches of propylene to the reactive catalyst lead to four isomeric products due to the C 2 symmetry of bridged Ind ligand of the catalyst. Consequently, various possibilities of propylene attack as well as orientation of polymer chain yield numerous stereoisomers. The calculations of the first and second insertions with various conformers clarified the most favorable reaction pathway and showed that isotactic propagation is more favorable (3.5 kcal mol ¹1 ) than syndiotactic propagation. The structures of resting state catalysts displayed various agostic interactions of the CH bond with the Zr center which stabilize the catalytic systems and play important roles in determining the favorable reaction pathway. The influence of counter anion¹ on the reactivity of the catalyst was also studied. The results also confirm that the trans orientation of the counter anion with respect to propylene is more favorable than its cis orientation and clarify the most favorable reaction pathway in the first and second insertion. Because agostic interactions are involved in various aspects, AIM analysis has been used to find the bonding nature of agostic interactions as well as ion-pair bonds. The overall results suggest that rigidity of ansa-zirconocene, unique structure of C 2 symmetric ansa ligand, influence of [CH 3 B(C 6 F 5 ) 3 ]¹ and β agostic interaction may restrict the attack of propylene only to isotactic polymerization and not to syndiotactic polymerization.
“…Kaminsky pointed out that the angle between the bridged ligand and its various substitutions can play a pivotal role for activity and stereospecificity. 15 This was verified by the experimental result that Et(2-Ind)(2-Ph-1-Ind)ZrCl 2 (Et = C 2 H 4 ) gave polypropylenes with higher isotacticity 16 ([mmmm] = 74%) than anti C 2 symmetric Me 2 Si(2-Ph-1-Ind) 2 ZrCl 2 ([mmmm] = 58%).…”
The [SiH 2 (Ind) 2 ZrCH 3 ] + (Ind = indenyl) catalyzed stereoregularity of propylene polymerization mechanism has been investigated at M06 level of theory. Four different approaches of propylene to the reactive catalyst lead to four isomeric products due to the C 2 symmetry of bridged Ind ligand of the catalyst. Consequently, various possibilities of propylene attack as well as orientation of polymer chain yield numerous stereoisomers. The calculations of the first and second insertions with various conformers clarified the most favorable reaction pathway and showed that isotactic propagation is more favorable (3.5 kcal mol ¹1 ) than syndiotactic propagation. The structures of resting state catalysts displayed various agostic interactions of the CH bond with the Zr center which stabilize the catalytic systems and play important roles in determining the favorable reaction pathway. The influence of counter anion¹ on the reactivity of the catalyst was also studied. The results also confirm that the trans orientation of the counter anion with respect to propylene is more favorable than its cis orientation and clarify the most favorable reaction pathway in the first and second insertion. Because agostic interactions are involved in various aspects, AIM analysis has been used to find the bonding nature of agostic interactions as well as ion-pair bonds. The overall results suggest that rigidity of ansa-zirconocene, unique structure of C 2 symmetric ansa ligand, influence of [CH 3 B(C 6 F 5 ) 3 ]¹ and β agostic interaction may restrict the attack of propylene only to isotactic polymerization and not to syndiotactic polymerization.
“… ,− We had proposed that the temperature and monomer concentration dependence of the stereospecificity of these catalysts is due to a competition between monomer insertion and conformational isomerization of the metallocene and proposed that the interconversion between enantiomeric anti conformers and at least one syn conformer was responsible for the generation of isotactic and atactic stereosequences (Scheme ). Indirect support for this proposal was provided by the polymerization behavior of the MAO-activated silyl-bridged rac ( anti ) and meso ( syn ) analogues, which produce isotactic and atactic polypropylene, respectively. , Recently, Busico has proposed, on the basis of microstructural analysis of these polymers, that the achiral syn isomer can be disregarded as a source of the atactic stereosequences and that the generation of isotactic and atactic stereosequences is the results of the interconversion of chiral anti isomers at a rate competitive with propylene insertion. According to this proposal, short atactic stereosequences are generated by kinetic states where the rate of conformational isomerization is faster than propylene insertion. − 1 Expected Low-Energy Conformations of [(2-PhInd) 2 Zr(polymeryl) + ] Based on Calculated Low-Energy Conformations of the Neutral Catalyst Precursor (2-PhInd) 2 ZrCl 2 a a For simplicity, the counteranion has not been depicted here. …”
The conformationally dynamic unbridged metallocene (2-PhInd) 2 ZrMe 2 (1) was activated with trispentafluorophenylborane (B(C 6 F 5 ) 3 , B2) or trityl tetrakispentafluorophenylborate ("trityl borate", [ 2b), respectively. Activation parameters for ion-pair separation were determined by line-shape analysis (2a: ∆H q ips ) 20 ( 1 kcal/mol, ∆S q ips ) 17 ( 4 eu; 2b: ∆H q ips ) 15 ( 2 kcal/mol, ∆S q ips ) 13 ( 7 eu). For 2a, a much slower B(C 6 F 5 ) 3 dissociation-reassociation process was also observed (∆G q 83°C ) 18.9 ( 0.1 kcal/ mol). Both 2a and 2b were treated with a series of o-substituted pyridines, and the behavior of the resulting zirconocenium-pyridyl complexes (3a-8b) was studied by 1 H, 13 C, and 19 F NMR over the temperature range -100 to 100 °C. Below room temperature, 1 H NMR NOESY spectra revealed signals characteristic of a C s -symmetric syn conformation.
“…The syntheses of these complexes as well as their propylene polymerization are described elsewhere. 40 Copolymerizations were carried out in neat R-olefin monomer with a constant overpressure of ethylene at 20 °C. Ethylene/propylene feed compositions were determined using gas fugacities as described by Kravchenko.…”
Ethylene/propylene and ethylene/1-hexene copolymerizations were studied with a series of
ansa-metallocenes characterized by a gauche orientation of the indenyl ligands: [1-(1-indenyl)-2-(2-indenyl)ethane]zirconium dichloride, Et(2-Ind)(1-Ind)ZrCl2 (1)/MMAO, [2-(2-indenyl)-1-(2-phenyl-1-indenyl)ethane]zirconium dichloride, Et(2-Ind)(2-Ph-1-Ind)ZrCl2 (2)/MMAO, and [(2-indenyl)(2-phenyl-1-indenyl)dimethylsilyl]zirconium dichloride, Me2Si(2-Ind)(2-Ph-1-Ind)ZrCl2 (3)/MMAO to determine the
significance of 2-phenyl ligand substitution as well as bridge type and placement on a series of gauche-1,2‘-bridged metallocenes. Comparison between the 1,1‘-bridged metallocene anti-ethylenebis(1-indenyl)zirconium dichloride and gauche-
1 showed that the different bridge placements gave similar copolymerization reactivity in contrast to the different reactivities observed for anti-1,1‘-bridged dimethylsilylbis(2-phenylindenyl)zirconium dichloride and gauche-1,2‘-dimethylsilyl-bridged 3 respectively. Similar to trends
observed with unbridged systems, substitution of a phenyl group in the 2-position resulted in a significant
increase in α-olefin incorporation for 2 compared to unsubstituted 1. Ethylene-bridged 2 gave higher
comonomer incorporation and a blockier comonomer distribution compared to dimethylsilylene-bridged
3.
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